134 MARINE ANGIOSPERMS [ch. 



tidal sea. Both the families to which the marine Angiosperms 

 belong, fulfil these four conditions in the persons of some, at 

 least, of their fresh-water representatives. The existence of such 

 species as Potamogeton pectinatus and Zannichellia palustris^'viKxch. 

 inhabit both fresh and brackish waters, and also of Ruppia and 

 Althenia (Potamogetonaceae), which typically occur in a brack- 

 ish medium as well as of Vallisneria spiralis'^ (Hydrocharita- 

 ceae) and Callitriche autumnalis'^ (Callitrichaceae), which are 

 able to tolerate some salt indicates how the transition from 

 fresh to saline water may have been bridged. The vegetative 

 organs, again, are entirely submerged in such genera as Elodea 

 and Vallisneria among the Hydrocharitaceae, and Zannichellia, 

 Naias and many Pondweeds among the Potamogetonaceae. 

 Both families also contain a number of species with well- 

 developed root systems. Finally, floating pollen is carried by 

 water to the stigmas in some species of Elodea and Ruppia, 

 while actual submerged pollination is found in Naias and 

 Zannichellia. These families are thus in every respect prepared, 

 as it were, for the evolution of marine members. The reason 

 why other families have not produced any forms adapted to life 

 in the sea, seems to be that, though certain of their species may 

 fulfil some of the conditions which we have enumerated, they 

 fail in others the one which is most rarely exhibited being a 

 tendency to sub-aquatic pollination. Myriophyllum spicatum and 

 Ranunculus Baudotii^, for example, have been observed to live 

 under conditions of slight salinity, but they are handicapped 

 for entry on marine life by the fact that they cannot be cross- 

 pollinated, unless the flowers are raised into the air. Cerato- 

 phyllum and Pseudo-callitriche, on the other hand, owing to 

 their hydrophilous pollination, suggest themselves as possible 

 candidates for marine life, but Ceratophyllum lacks roots en- 

 tirely, and Pseudo-callitriche has no rhizome obstacles that 

 may well prove insuperable. Conceivably in future ages, if the 



1 Chatin, A. (18552). 2 Lgbgi^ , (1863). 



3 Ostenfeld, C. H. (1908). 



